Electrokinetic processes have been shown to be useful in the decontamination of soils and industrial sludges. Typically, a voltage gradient is applied between an anode and a cathode so that an electric current is produced. The current induces water, ions and charged particles to migrate towards the anode or the cathode. Water flows from the anode toward the cathode by a process known as electroosmosis in soils containing negative surface charged particles. The process is expected to reverse direction when the soil contains predominantly positively charged particles. The flow of water transports particles that are dissolved or suspended in the water. In addition, anions or negatively charged species migrate toward the anode, while cations or positively charged species migrate toward the cathode. Further, positively and negatively charged colloidal particles are also induced to migrate toward the cathode and the anode, respectively, by the electrical current. As a result, positively and negatively charged species can be concentrated around the cathode and the anode for subsequent removal.
Electrokinetics has been used to improve and reclaim land by lowering the water content and subsequently densifying the soil in high water content areas and also facilitating the removal of unwanted salts from the soil. It is well known that certain soils contain excessive amounts of saline or alkali salts and that these salts can substantially reduce the agricultural utility of the soil. Typically, a cathode is placed along the bottom of a drainage ditch. A series of anodes are driven into the soil in a line generally parallel to the ditch. When an electric current is applied, an increase in the amount and the salinity of the water in the drainage ditch is observed. Alternatively, electrokinetic processes can be used to enhance the removal of contaminants from soils through leaching. Porous electrodes can be utilized so that a leaching fluid, such as water, is supplied to the anode. The electric current induces the fluid to flow toward the cathode. The flow of fluid causes the contaminants to collect near the cathode where they can be removed through the pores in the cathode by pumping.
In addition, electrokinetic processes have been used to enhance the settling rate, filterability, and dewatering of industrial sludge. Direct dewatering of the sludge is achieved by passing an electrical current through the sludge between a pair of perforated electrodes. Water flows toward the cathode due to electroosmosis and is collected as it passes through the perforated cathode. In addition, ionic particles accumulate near the anode as a result of electrophoresis, where they can be collected as they pass through the perforated anode.
However, electrochemical/electrokinetic processes have heretofore not been used in treating nuclear fuel reprocessing liquid tank, and cake wastes. These wastes typically have a high salt content and are contaminated with fission products which have long half-lives such as plutonium, cesium, and the actinides. These waste products are preferably transformed into a suitable, stable waste form before disposal to minimize the potential for environmental contamination.
One such waste form is produced when a solid state radioactive waste material is bonded to a cathode by electrodepositing a corrosion-resistant metal, such as copper, onto the cathode in the presence of the waste material. The waste material is thus incorporated into a matrix of metal as the metal is electrodeposited onto the cathode. This technique requires that the waste be in a suitable form, such as radionuclide containing oxide, oxyhydroxide, or hydroxide particles. The waste must be essentially insoluble in the electrolytic solution used for the electrodeposition and must be capable of being brought into contact with the cathode. This is often accomplished by forming the waste into a suitable size and shape, such as a pellet, or by coating or mixing the waste with a conductive material.
Another such suitable waste form would be a non-leaching vitrified product, such as a glass or ceramic. Techniques for vitrifying waste are generally known, and examples of methods for vitrification of contaminated soil are described in an application for a U.S. patent Ser. No. 08/266,209 (Wittle et al.) and a Department of Energy Report (PNL-8525, June 1993), which are both incorporated by reference in the present application as if set forth herein in full. A problem arises, however, since the amount of salt which can be stabilized in a vitreous product is limited. This is a formidable problem since much of the radioactive waste, such as that produced by nuclear fuel reprocessing, contains unacceptably high salt contents. It is therefore necessary to remove the salt from the waste while retaining the fission products within the waste before the waste is vitrified.
Accordingly, a system and method having the capability of removing charged species from contaminated liquids and porous solids is a highly desirable objective. In particular, the system and method should be applicable to the removal of various salts from industrial sludges, contaminated soils, and nuclear wastes which are commingled with radionuclides. The system and method should enable the selective removal of the salts while maintaining the radionuclides within the sludge, soil, or waste for conversion to an immobilized form. In addition, the system and method should be efficient, cost effective, and applicable to a wide range of waste materials.